Multiscale, multifunction diffractive structures wet etched into fused silica for high-laser damage threshold...

Area of Science:

• Optics and Photonics
• Materials Science
• Nanofabrication

Background:

• Diffractive optics are crucial for manipulating light, but integrating components with significant scale differences presents fabrication challenges.
• Fabricating micro- and nanostructured optical elements often requires precise control over etching processes to maintain feature integrity.

Purpose of the Study:

• To demonstrate a method for combining two diffractive optical elements with a ~100x scale difference on a single substrate.
• To investigate the preservation of fine grating features during the fabrication of larger, deeper grating structures using wet etching.

Main Methods:

• Interference lithography was used to pattern fine-scale (2-µm-period) gratings in photoresist.
• Hydrofluoric acid etching transferred the fine gratings to fused silica.
• A two-mask proximity printing and wet etching process created larger (115-µm-linewidth) stairstep gratings for color separation.

Main Results:

• The fine-scale lamellar gratings' shapes were well-preserved after etching the deeper, larger color separation gratings.
• Model simulations accurately predicted the shape evolution of the gratings during isotropic etching.
• A rule of thumb was established: lamellar grating profiles can be etched to ~0.08A⁻² times their depth before shape degradation.

Conclusions:

• This fabrication approach successfully integrates diffractive optics with large scale disparities on a single surface.
• The study highlights the importance of the initial aspect ratio in maintaining grating profile fidelity during anisotropic etching.
• The findings provide valuable insights for designing and fabricating complex diffractive optical elements.